This invention relates in general to mass transfer and exchange columns and, more particularly, to downcomers used in association with vapor-liquid contact trays employed in such columns.
Vapor-liquid contact trays are used in mass transfer or heat exchange columns to facilitate interaction and mass transfer between descending liquid streams and ascending vapor streams. The trays arc generally horizontally disposed and vertically spaced apart within an open interior region of the column. Each tray typically includes a flat deck portion that includes a plurality of vapor flow apertures that allow ascending vapor to pass through the tray deck and interact with liquid flowing across the upper surface of the tray deck. A downcomer is positioned at an opening at an outlet end of the tray deck to remove liquid from the deck and direct it downwardly to a liquid receiving area at the inlet end of an underlying tray. The liquid then flows across the tray deck of the underlying tray, interacts with vapor passing through the tray deck, and then flows downwardly through the associated outlet downcomer to the next underlying tray. This flow pattern is then repeated for each successively lower tray.
In conventional columns where high liquid flow rates are encountered, it has been suggested in U.S. Pat. No. 5,213,719 that a second downcomer can be used on each tray to increase the liquid handling capacity of the tray and thereby reduce the opportunity for flooding of the downcomer to occur. The second downcomer, referred to as the upstream downcomer, is positioned adjacent to the downstream downcomer and is shorter than the downstream downcomer in vertical length. FIG. 1, taken from U.S. Pat. No. 5,213,719, illustrates this downcomer construction with the upstream downcomer 10 and downstream downcomer 12 positioned at the outlet end of the tray deck 14.
It has also been suggested, in U.S. Pat. No. 5,453,222, that the normally planar downcomer inlet wall can be shaped in a semi-conical fashion to form a vapor tunnel along the undersurface of the semi-conical wall. The vapor tunnel imparts a horizontal flow vector to the vapor stream and facilitates disengagement of liquid from the vapor stream. FIGS. 2 and 3 are taken from U.S. Pat. No. 5,453,222 and illustrate a tray 16 with a downcomer 18 having a semi-conical inlet wall 20. Venting chambers 22 positioned in the liquid receiving trough 24 on the underlying tray 26 allow vapor to flow through the chambers 22 for upward passage through the overlying vapor tunnel 28 formed by the semi-conical downcomer inlet wall 20.
It would be desirable to combine the advantages afforded by the double downcomer disclosed in the above-mentioned U.S. Pat. No. 5,213,719 with those provided by a downcomer with a semi-conical inlet wall as taught by U.S. Pat. No. 5,453,222 discussed above. Several problems, however, would result from such a combination because the upstream downcomer would need to be of a relatively short vertical dimension so that it does not protrude downwardly into the vapor tunnel and interfere with the desired flow of vapor through the vapor tunnel. If a relatively short upstream downcomer is used, liquid issuing from the bottom of the upstream downcomer would be discharged directly into the vapor stream flowing along the vapor tunnel. The momentum of the vapor stream would cause the discharged liquid to be blown away from the downcomer and across the tray. The vapor-liquid contact and energy and mass exchange occurring in such blowing liquid as it moves through the vapor is not as good as is to be desired. In addition, the blown liquid would bypass portions of the tray deck and would not experience the vapor-liquid interaction that would otherwise occur if the liquid flowed completely across the tray deck. Therefore, it is desirable to minimize or eliminate this effect.
Another untoward effect which may occur as a consequence of utilizing an upstream downcomer is it may "starve" liquid flow from the downstream or primary downcomer under low flow conditions. A further consequence of this effect is that the downstream downcomer may have too little liquid flowing through it and it may lose the liquid seal at the bottom region of the downcomer that blocks undesired entry of vapor into the downcomer. Loss of the liquid seal will allow vapor to flow upwardly through the downcomer and bypass interaction with liquid on the overlying tray. The possibility that such an effect will occur decreases the operating flexibility of the column taken as a whole.
A still further undesirable result from the use of an upstream downcomer of small vertical extent is that liquid issuing from the bottom of the upstream downcomer falls in free-fall vertically downward to the tray deck below. The large momentum of the falling liquid is transmuted into pressure when the liquid hits the tray below and locally depresses the vapor flow in the impact area and, in consequence, allows the liquid to weep through the vapor apertures at that point in the tray deck.
While the foregoing undesirable effects of utilizing an upstream downcomer of short vertical extent have been described in connection with a downcomer system utilizing a vapor tunnel structure, those skilled in the art will appreciate that these undesirable effects can also be encountered when the upstream downcomer is of slight vertical extent, even if there is no vapor tunnel. It would thus be desirable to overcome these disadvantages in a double downcomer system.